Because too much contrast on a monitor means white on black, which on average (that is, 99% of people's setups) brightness levels settings strains the eyes and people actively dislike it.

Its not some book reflecting ambient light, but a monitor with a led shining white (blue-ish) light on your eyes...

>Contrast is good.

Water is good too. Too much water though, and you die: https://en.wikipedia.org/wiki/Water_intoxication